The present invention generally relates to a broaching tool for gear forming. More particularly, the present invention relates to a broaching tool constructed of a plurality of wafers for finish cutting helical splines on the interior of an annular workpiece.
Broaching is a machining process in which a cutting tool that has multiple transverse cutting edges is pushed or pulled through a hole or over a surface to remove metal by axial cutting. Typically, a broach is a tapered bar formed from bar stock, into which teeth have been cut so as to produce a desired contour in a workpiece by a single pass of the tool.
A conventional prior art broaching bar for cutting a round hole in a workpiece is shown schematically in FIG. 1a, identified with reference numeral 1. Such a conventional broaching bar 1, includes three basic sections of teeth: roughing teeth 2, intermediate or semifinishing teeth 3 and finish teeth 4. Broaching bar 1 further includes a pull end 5 and a following end 6.
In use, a workpiece to be internally broached must be provided with a starting hole through which the broaching bar is pushed or pulled. Most internal broaching is accomplished by pulling the broach through the hole. With pulling there is no problem of bending, thereby allowing pull broaches to be longer than push broaches for the same size hole. The hole in the workpiece should be just large enough to permit a front pilot 7 section of the broaching bar 1 to enter freely. As the broaching bar progresses through the part, cutting commences gradually, and as each succeeding tooth engages the work it removes a small amount of metal. Since both roughing and finishing can be accomplished in a single pass of the broaching bar, broaching has proven rapid and efficient.
The limitations of broaching stem from the fundamental characteristics inherent in the process. Broaching bars are very expensive tools, and any revision in the design of a gear, such as a change in the number of teeth or the pitch diameter, would require a new broaching bar. Wear or abuse, often as simple as broken teeth, eventually necessitates replacement of the entire broaching bar.
An improvement to conventional broaching bars is described in detail in U.S. Pat. No. 4,274,767 to Bistrick, et al., entitled "Helical Toothed Broach." The broach as described in U.S. Pat. No. 4,274,767 includes a plurality of circumferentially disposed groups of cutting teeth which are integrally formed on a removable shell. Such a conventional broaching bar adapted to incorporate a removable shell is illustrated in FIG. 1b. Typically, broaching bar 1 includes roughing teeth 2 and intermediate teeth 3, and is modified to include an arbor section 8, over which the removable shell 9 fits.
Prior art broaching bars which have heretobefore been modified to include removable shells have proven to be an advance over conventional broaching bars since worn or broken teeth on the shells 9 can be replaced without replacing the entire broaching bar. However, none of such prior art is without its disadvantages. In this regard, conventional broaching tools limit the "form relieving" processes which can be used in manufacturing the finished profile for the broaching tool. Additionally, conventional broaching tools are extremely expensive in that damaged shells must be replaced in their entirety. Further, the angles of the cutting edges of conventional broaching tools are limited by having the rows of teeth integrally connected.